In an FMCW radar system, the transmitter produces a transmitted signal having a frequency that varies linearly with time. The transmitted signal is derived from a voltage controlled oscillator (VCO), and the input signal to the VCO is controlled to produce the linear frequency sweep. The return signal from a target is mixed with the transmitted signal to produce an intermediate frequency (IF) signal having a frequency equal to the difference between the instantaneous frequency of the transmitted signal and the frequency of the return signal. Because the frequency of the transmitted signal changes linearly with time, the frequency of the IF signal is a function of target range.
In one important class of applications, an FMCW radar system is mounted in an aircraft and scans the ground at a location ahead of the aircraft. In such a system, it is necessary to isolate and analyze the "ground return," i.e., the energy reflected from the ground at the spot on which the radar beam is focused. Analysis of the ground return provides the range to the scanned spot on the ground. In addition, the ground return may be analyzed to identify radar signatures of particular objects or types of objects within the illuminated spot.
In order to isolate the ground return, the IF signal produced by the FMCW radar is input to a "range tracker" that is essentially a signal bandwidth limiter. The range tracker takes the IF signal having a large bandwidth (or range extent), and isolates and tracks the smaller bandwidth ground return portion of the signal. The bandwidth of the IF signal must be limited because the subsequent processing stages, i.e., the stages that search for particular radar signatures, have input bandwidth limitations that are much less than the bandwidth of the IF signal. For example, at typical altitudes and look down angles, a typical airborne FMCW radar system may produce a ground return that is approximately 1-3 MHz wide. This band of frequencies can exist anywhere between the range limits of the radar, which may for example be 1-100 MHz. Thus the ground return typically occupies only about 3% of the bandwidth of the IF signal.
Prior range trackers have generally been of two types: power sensors, or past history systems. Power sensors track the band of frequencies with the highest power return, which band is assumed to be the ground return. Temporary loss of signal strength means the system must search the range extent over which the radar operates, and reacquire the target. Generally, a time delay is applied such that the search and acquire is not initiated instantly upon loss of the ground return. Thus using power sensors, there can be a significant blind period during which the radar does not know the location of the ground return.
Past history systems offer a more elegant solution than power sensors. A past history system relies on the recent history of the radar return signal, and complex prediction techniques, to fill in short signal dropouts. Such systems are effective, but very complex, and require large amounts of data storage in order to accumulate past history information. There is therefore a need for an FMCW radar system that is less complex than a past history system, but that can reacquire the ground return more rapidly than power sensor systems.